1. Field of the Invention
The present invention relates to a method and an apparatus for transmitting/receiving an ACK/NACK according to a method for multiplexing downlink control information and data in an OFDMA (Orthogonal Frequency Division Multiple Access) wireless communication system.
2. Description of the Related Art
In the field of mobile communication systems, the OFDM (Orthogonal Frequency Division Multiplexing) scheme has recently been studied widely as a scheme useful for high-speed data transmission through wireless channels.
The OFDM scheme employs multi-carriers to transmit data. Particularly, the OFDM is a type of multi-carrier modulation scheme, which converts serially inputted strings of symbols into parallel symbols, modulates the respective parallel symbols into a number of sub-carriers having orthogonality relative to one another, i.e. a number of sub-carrier channels, and transmits them.
FIG. 1 shows the structure of a transmitter of a conventional OFDM system.
Referring to FIG. 1, the OFDM transmitter includes an encoder 101, a modulator 102, a serial/parallel converter 103, an IFFT block 104, a parallel/serial converter 105, and a CP inserter 106.
The encoder 101, also referred to as a channel encoding block, receives a string of information bits as an input and conducts channel encoding. A convolutional encoder, a turbo encoder, or an LDPC (Low Density Parity Check) encoder is conventionally used as the encoder 101.
The modulator 102 conducts modulation, such as QPSK (Quadrature Phase Shift Keying), 8PSK (Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation), 64QAM, 256QAM, etc.
Those skilled in the art can easily understand that, although not shown in FIG. 1, a rate matching block may be added between the encoder 101 and the modulator 102 to conduct repetition, puncturing, etc.
The serial/parallel converter 103 receives the output of the modulator 102 as an input and converts the signal into a parallel signal.
The IFFT (Inverse Fast Fourier Transform) block 104 receives the output of the serial/parallel converter 103 as an input and conducts IFFT operations. The output of the IFFT block 104 is converted by the parallel/serial converter 105.
The CP inserter 106 inserts a CP (Cyclic Prefix) into the output signal of the parallel/serial converter 105.
In order to transmit packets, the Node B of the OFDMA-type communication system allocates a suitable resource to a UE by means of scheduling. Then, the UE transmits/receives data by using the resource and, according to whether or not the data has errors, retransmits an ACK/NACK signal as a response to the data based on an HARQ (Hybrid Automatic Repeat reQuest). Operations for transmitting packet data by the communication system will now be described with reference to FIG. 2.
FIG. 2 shows a procedure for transmitting downlink packet data.
Referring to FIG. 2, the Node B 201 receives UE condition information, particularly power information, or data buffer information from a plurality of UEs, including a UE 202, in step 203. The Node B 201 schedules UEs belonging to the Node B 201 by using information regarding the condition of the UE 202 in step 204. When a downlink transmission to the UE 202 is determined based on the scheduling, the Node B 201 transmits scheduling control information and packet data to the UE 202 in steps 205 and 206. The scheduling control information refers to information regarding control of the packet data, and may include the construction of wireless resources or data used to transmit the packet data, the transmission scheme, the HARQ information, etc. The scheduling control information is transmitted either concurrently with or prior to the transmission of the packet data. After receiving the scheduling control information and the packet data, the UE 202 decodes the received packet data by using the scheduling control information, and determines if the packet data has been successfully received without errors in step 207. If it is determined in step 207 that the packet data has errors, NACK information is transmitted, and, if no errors have occurred, ACK information is transmitted to the Node B 201 by means of ACK/NACK signaling in step 208. After receiving the ACK/NACK, the Node B 201 evaluates the ACK/NACK in step 209. If an ACK has been transmitted, the transmission of packet data is terminated, and if an NACK has been transmitted, the corresponding packet data is retransmitted in step 211. In this case, control information regarding the retransmitted packet data may be transmitted simultaneously. Alternatively, the scheduling control information may be transmitted prior to the retransmitted packet data as in step 210. The retransmitted packet data includes the same information as the packet data transmitted in step 206, but the transmission type may vary depending on the AMC (Adaptive Modulation and Coding) conducted by the Node B 201.
FIG. 3 shows a procedure for transmitting uplink packet data.
Referring to FIG. 3, the Node B 301 receives UE condition information, particularly power information, or data buffer information from a plurality of UEs, including a UE 302, in step 303. The Node B 301 schedules UEs belonging to the Node B 301 by using the UE condition information in step 304. When an uplink transmission to the UE 302 is determined based on the scheduling, the Node B 301 transmits scheduling control information to the UE 302 in steps 305. The scheduling control information refers to information regarding control of uplink packet data, the transmission of which to the UE 302 has been allowed, and may include the construction of wireless resources or data used to transmit the uplink packet data, the transmission scheme, the HARQ information, etc. After receiving the scheduling control information, the UE 302 transmits packet data to the Node B 301 by using the transmission scheme and wireless resources, which have been allocated to it based on the scheduling control information, in step 306. After receiving the packet data, the Node B 301 determines if the packet data has been successfully received without errors in step 307. If it is determined in step 307 that the packet data has errors, NACK information is transmitted, and, if no errors have occurred, ACK information is transmitted to the UE 302 by means of ACK/NACK signaling in step 308. After receiving the ACK/NACK, the UE 302 evaluates the ACK/NACK. If an ACK has been transmitted, the transmission of the packet data is terminated, and if an NACK has been transmitted, a retransmission of the packet data is prepared. The Node B 301 reschedules the retransmission of the packet data in step 309, and transmits scheduling control information for the retransmission to the UE 302 in step 310. After receiving the scheduling control information, the UE 302 retransmits packet data to the Node B 301 in step 311. The retransmitted packet data includes the same information as the packet data transmitted in step 306, but the transmission type may vary depending on the AMC conducted by the Node B 301.
A method for multiplexing the ACK/NACK occurring in the process for transmitting downlink or uplink packet data will now be described. In general, the ACK/NACK includes one-bit information or a small amount of information. Therefore, it is preferred in terms of resource efficiency to transmit the ACK/NACK using the smallest amount of resources. However, the characteristics of OFDM systems require that, in order to improve the frequency diversity effect, the ACK/NACK must be distributed over a wide frequency band and then transmitted so as to guarantee the ACK/NACK performance having a high level of error requirements. In the case of a frequency multiplexing method, which solely transmits a signal regarding one UE by using one frequency resource, many frequency resources must be allocated to a single UE in order to transmit a single ACK/NACK. This degrades the resource efficiency. Therefore, an inter-UE code multiplexing method can be used to transmit the ACK/NACK. According to the code multiplexing method, a plurality of UEs share the same resources, but use different codes so that signals can be differentiated among the UEs. The code multiplexing method ensures that all UEs equally have the frequency diversity effect while maintaining a high level of resource efficiency.
A method for multiplexing the ACK/NACK in the downlink and uplink will be described with reference to FIGS. 4 and 5, respectively.
Referring to FIG. 4, the downlink ACK/NACK is characterized in that a single Node B transmits information regarding a number of UEs as a whole. It is assumed in FIG. 4 that the Node B transmits the ACK/NACK with regard to n UEs. The ACK/NACK signals 401-404, which are to be transmitted to the n UEs from the Node B, pass through a unitary precoder 405 so that they are converted into orthogonal codes and are multiplexed. The output of the unitary precoder 405 is inputted to a resource mapper 406 so that it is mapped to time-frequency resources, which are to be used to transmit the multiplexed ACK/NACK signals, and is transmitted to the n UEs. The shaded portions 408 in FIG. 4 correspond to those of the entire time-frequency resources 407, which are used to transmit the ACK/NACK signals. It is clear from FIG. 4 that, when a plurality of UEs share resources 408 during a downlink ACK/NACK transmission, the resource utilization efficiency improves, and all UEs can have frequency diversity gain.
Referring to FIG. 5, the uplink ACK/NACK is characterized in that a single UE transmits a signal to a single Node B. It is assumed in FIG. 5 that each of n UEs transmits its ACK/NACK to a single Node B. The n UEs encode their ACK/NACK signals 501-503 and transmit them by using the same resources. More particularly, the UE 1 encodes an ACK/NACK 501 with an encoder 504, maps it to a resource 511, which has been selected from the entire time-frequency resources 510 in order to transmit the ACK/NACK, by using a resource mapper 505, and transmits the ACK/NACK to the Node B. The UE 2 encodes an ACK/NACK 502 by an encoder 506, maps it to a resource 511, which has been selected from the entire time-frequency resources 510 in order to transmit the ACK/NACK, by using a resource mapper 507, and transmits the ACK/NACK to the Node B. Similarly, the UE n encodes an ACK/NACK 503 by an encoder 508, maps it to a resource 511, which has been selected from the entire time-frequency resources 510 in order to transmit the ACK/NACK, by using a resource mapper 509, and transmits the ACK/NACK to the Node B. The plurality of ACK/NACKs transmitted by the plurality of UEs in FIG. 5 are code-multiplexed, because they are encoded, mapped to the same resources, and transmitted. It is clear from FIG. 5 that, when a plurality of UEs share resources 511 during an uplink ACK/NACK transmission, the resource utilization efficiency improves, and all UEs can have frequency diversity gain.
The conventional method for code-multiplexing the ACK/NACK, which has been described above, is advantageous in terms of resource efficiency and frequency diversity. However, such code multiplexing often fails to perfectly satisfy the orthogonality in the case of a channel having a high level of channel selectivity. Particularly, when the code of each ACK/NACK has a varying degree of receiving power intensity, an ACK/NACK using a code with low receiving power may have very poor receiving performance due to interference with another ACK/NACK using a code having high receiving power.